The europium halide is generally known as a material for preparing an europium activated phosphor. For example, a phosphor screen produced by a process disclosed in WO No. 01/0356 A1 comprises a substrate and a stimulable europium-activated cesium halide phosphor layer formed thereon by a two vapor source deposition process in which a cesium halide and an europium compound such as an europium halide are used as evaporation sources. In the disclosed deposition process, the cesium halide (which is a matrix component of the phosphor) and the europium compound (which is an activator component of the phosphor) are simultaneously heated, vaporized, and reacted with each other to form the phosphor, which is immediately deposited and accumulated on the substrate to form a phosphor layer.
The europium halide generally is in the form of powder and is hygroscopic. If the powdery europium halide absorbs water from the surrounding atmosphere, the water contained in the powdery europium halide is liable to cause bumping in the evaporation-deposition process. Accordingly, it is difficult to form a deposited layer having a uniform phosphor composition.
Further, when the europium halide is heated in an atmosphere containing water vapor or gaseous oxygen, not a small amount of an europium oxyhalide is produced. It is, therefore, difficult to obtain a pure fused solid of an europium halide having little europium oxyhalide by merely heating and fusing the powdery europium halide.
There are known various stimulable phosphors. When the stimulable phosphor is exposed to radiation such as X-rays, it absorbs and stores a portion of the radiation energy. The stimulable phosphor then emits light according to the level of the stored energy when exposed to stimulating rays. A radiation image storage panel (often referred to as stimulable phosphor sheet) is a sheet comprising the stimilable phosphor, and has a basic structure comprising a support and a stimulable phosphor layer.
If the stimulable phosphor layer of the storage panel is formed by the vacuum deposition process, it consists essentially of the stimulable phosphor alone. In that phosphor layer, there are cracks among columnar crystals of the phosphor. Because of the cracks, the stimulating rays can stimulate the phosphor efficiently and the emitted light can be collected efficiently, too. Consequently, the storage panel having the vacuum deposited phosphor layer has high sensitivity. In addition, since the cracks prevent the stimulating rays from diffusing parallel to the layer, the storage panel can give a reproduced image with high sharpness.
The radiation image storage panel is advantageously employed in a radiation image recording and reproducing method, which has been widely used in practice. The method comprises the steps of causing the stimulable phosphor of the storage panel to absorb radiation energy having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the panel; sequentially exciting the stimulable phosphor with a stimulating rays such as a laser beam to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric signals giving the image information.
The applicants have now found that, if the phosphor layer of the panel is formed by the vapor deposition process in which europium halide containing not a small amount of an europium oxyhalide is used as the evaporation source, the vaporization rate (i.e., a rate of vapor flow emitted from the source) is rendered so unstable that the deposited layer cannot have a uniform phosphor composition. Further, since the europium halide is contaminated with the impurities, a pure phosphor layer cannot be produced. A radiation image storage panel having the impure phosphor layer is liable to give a radiation image of poor quality.
Further, if the content of water in the europium halide is high, troubles such as bumping are often observed in the evaporation-deposition process, and furthermore, the partial pressure of water increases to make the deposition atmosphere so unstable that columnar crystals of the phosphor in the resultant layer are formed in undesirable shapes and that the emission is not evenly given off of the whole surface of the layer (“uneven emission”) when the whole phosphor layer is stimulated.